P21C3Reductant Heater Relay Control Circuit Low
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- P21C3 is the low-side fault on the control line of the DEF heater relay: the signal is pulled toward ground, for example by a chafed wire touching the frame or a shorted relay coil, so the relay no longer follows the command. Ford's 2023 diesel summary uses the same continuous check as for P21C2, but here the relay stage is on, every heater has been told to stay off, and still more than 6 A of heater current is measured; the fault is confirmed within 5 s. Such a reading means power reaches the heater circuit by a path the module never requested. Whether the heaters then stay dead or run without control depends on how the relay is wired. P21C2 is the open-circuit counterpart, P21C4 the high-side fault, P21C7 the module power relay.
- What you may notice
- Check engine light on
- DEF heating inactive or running when not commanded
- Blown fuse in the heater supply
- Likely causes
- LikelyControl wire chafed through and shorted to ground
- PossibleRelay coil with internal short or welded contacts
- PossibleWater or salt in the relay or heater connector
- RareOutput stage of the controlling module faulty
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Usually a job for a workshop.Typical cost: €30 – €500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.